High frequency amplifier

ABSTRACT

Provided is a high frequency amplifier including two amplifying elements of different element sizes connected in parallel and switching the amplifying elements in accordance with a level of output power. In particular, the high frequency amplifier includes an output matching circuit for matching to characteristic impedance (50 ohms) both when the output power is high and low, and increasing impedance when the turned-off amplifying element is viewed from a connection node on an output side of the two amplifying elements. Consequently, characteristics such as high output power and high efficiency can be achieved and it is possible to prevent an amplified high frequency signal from passing around to a matching circuit on a turned-off amplifying element side.

TECHNICAL FIELD

The present invention relates to a high frequency amplifier in which twoamplifying elements having different sizes are connected in parallel toeach other, and the amplifying elements are switched in accordance withhigh or low output power. In particular, the present invention relatesto an output matching circuit that is matched to be a characteristicimpedance (50 ohms) in any cases of high and low output powers so thatthe impedance of a turned-off amplifying element viewed from anoutput-side connection node of the two amplifying elements is set to bea high value.

BACKGROUND ART

The high frequency amplifier usually has such characteristics that theefficiency increases as the output level becomes close to a saturationlevel. On the contrary, there is a problem in that the efficiency is lowwhen the output level is low. For instance, if the high frequencyamplifier is used in a system having a wide dynamic range of the outputpower, the efficiency in a low output power becomes low. In this case,it is a task to enhance the efficiency in a low output power.

A conventional high frequency amplifier is devised to enhance theefficiency in a low output power like a high frequency amplifierdisclosed in Non Patent Document 1, for example, by connectingamplifiers having different sizes in parallel to each other and byswitching the amplifiers in accordance with the output level so that anamplifier having a larger size is activated if the output level is highwhile the other amplifier having a smaller size is activated if theoutput level is low.

In addition, Patent Document 1 discloses a method of switching a size ofthe amplifier by using a switch made up of transistors.

In addition, Patent Document 2 discloses an output matching circuit inwhich an output impedance of an amplifier is matched to be acharacteristic impedance of 50 ohms (Ω) in any case when the amplifieris switched.

In addition, Patent Document 3 discloses a devised structure forenhancing the efficiency in a low output power by controlling acollector voltage of an amplifier. It also discloses a devised structurefor enhancing the efficiency in a low output power by changing an outputmatching circuit with a switch simultaneously when a size of theamplifier is changed.

In addition, Patent Document 4 discloses a devised structure forenhancing the efficiency in a low output power by switching an outputmatching circuit with a switch when a size of the amplifier is changed.

Further, Patent Document 5 discloses a devised structure of an amplifierin which sizes of two stages of amplifiers are switched in accordancewith an output level. A switch is provided between the stages ofamplifiers, and a switch provided to the amplifier to be turned off isswitched off so that isolation is enhanced for suppressing anoscillation.

Patent Document 1: JP 2000-278109 A

Patent Document 2: JP 2003-046340 A

Patent Document 3: JP 2002-353751 A

Patent Document 4: JP 2004-134823 A

Patent Document 5: JP 2003-087059 A

Non Patent Document 1: J. H. Kim, etc., “A Power Efficient W-CDMA SmartPower Amplifier With Emitter Area Adjusted For Output Power Levels”,2004 IEEE International Microwave Symposium (MTT-S) Digest, pp.1165-1168.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The conventional high frequency amplifiers disclosed in Non PatentDocument 1 and Patent Document 1 have a problem that an output loadimpedance is not optimized so that the characteristics are deterioratedwhen the amplifier is switched, because matching circuits for the twoamplifiers are the same matching circuit. In addition, there is aproblem that an impedance of the turned-off amplifier affects a loss inthe output matching circuit to increase, and hence characteristics suchas the output power and the efficiency are deteriorated. Further, thereis a problem that the high frequency signal may pass around theturned-off amplifier and cause an oscillation because of insufficientisolation of the turned-off amplifier.

Patent Document 2 describes that the conventional high frequencyamplifier is matched to have the output characteristic impedance whenthe amplifier is switched in any case. However, it has a problem thatthe impedance of the turned-off amplifier affects the loss in the outputmatching circuit to increase so that the characteristics such as theoutput power and the efficiency are deteriorated. In addition, there isa problem that the high frequency signal may pass around the turned-offamplifier and cause an oscillation because of insufficient isolation ofthe turned-off amplifier.

Patent Documents 3 and 4 describe that the matching circuit is alsoswitched with the switch simultaneously when the amplifier is switched,and hence it is matched to the output characteristic impedance in anycase. However, the switch is used, and hence there are problems that acircuit size increases and that a loss in the switch causes an increaseof a loss in the output matching circuit so that the characteristicssuch as the output power and the efficiency are deteriorated. Inaddition, there is a problem that the impedance of the turned-offamplifier affects a loss in the output matching circuit to increase sothat the characteristics such as the output power and the efficiency aredeteriorated. Further, there is a problem that the high frequency signalmay pass around the turned-off amplifier and cause an oscillationbecause of insufficient isolation of the turned-off amplifier.

The conventional high frequency amplifier described in Patent Document 5includes the amplifier to be switched that is made up of two stages ofamplifiers and the switch disposed between the two stages of amplifiers.When the amplifier is turned off, the switch is also turned off so thatsufficient isolation can be obtained. Thus, it is avoided that the highfrequency signal passes around the turned-off amplifier and causes anoscillation. However, the switch is provided, and hence there is aproblem that the circuit size increases. In addition, there is a problemthat the output load impedance is not optimized so that thecharacteristics are deteriorated when the amplifier is switched.Further, there is a problem that the impedance of the turned-offamplifier affects a loss in the output matching circuit to increase sothat the characteristics such as the output power and the efficiency aredeteriorated.

The present invention has been made to solve the problems describedabove, and therefore an object thereof is to obtain a high frequencyamplifier that can be matched to a characteristic impedance of 50 ohms(Ω) in any cases of high and low output powers, and hence as to realizecharacteristics such as high output power and high efficiency.

In addition, it is an object to obtain a high frequency amplifier thatcan prevent the amplified high frequency signal from passing around theturned-off amplifying element to the matching circuit, can reduce a lossin the output matching circuit, can enhance isolation between the inputand the output of the turned-off amplifying element side, and cansuppress the oscillation due to the signal passing around the turned-offamplifying element.

Means for Solving the Problems

A high frequency amplifier according to the present invention includes:a first amplifying element for amplifying a high frequency signal inputfrom an input terminal; a second amplifying element for amplifying thehigh frequency signal, which is connected in parallel to the firstamplifying element and has a smaller element size than the firstamplifying element has; a first bias control circuit for turning on andoff the first amplifying element based on a mode switching voltage forswitching between a case where an output power is high and a case wherethe output power is low; a second bias control circuit for turning onand off the second amplifying element based on the mode switchingvoltage; and an output matching circuit connected to output sides of thefirst amplifying element and the second amplifying element. The outputmatching circuit includes: a first matching circuit connected to theoutput side of the first amplifying element; a second matching circuitconnected to the output side of the second amplifying element; and athird matching circuit connected between an output terminal and aconnection node of the output sides of the first matching circuit andthe second matching circuit, which is matched to 50 ohms. The firstmatching circuit comprises: a first high pass filter type matchingcircuit connected to the output side of the first amplifying element;and a serial inductor connected to the first high pass filter typematching circuit. The second matching circuit comprises a second highpass filter type matching circuit connected to the output side of thesecond amplifying element. A first impedance of the first matchingcircuit viewed from the connection node in the case where the outputpower is high that is a case where the first amplifying element isturned on while the second amplifying element is turned off issubstantially the same as a second impedance of the second matchingcircuit viewed from the connection node in the case where the outputpower is low that is a case where the second amplifying element isturned on while the first amplifying element is turned off. The secondimpedance of the second matching circuit viewed from the connection nodeis higher than the first impedance of the first matching circuit viewedfrom the connection node in the case where the output power is high thatis in the case where the first amplifying element is turned on while thesecond amplifying element is turned off. The first impedance of thefirst matching circuit viewed from the connection node is higher thanthe second impedance of the second matching circuit viewed from theconnection node in the case where the output power is low that is thecase where the second amplifying element is turned on while the firstamplifying element is turned off.

EFFECTS OF THE INVENTION

The high frequency amplifier according to the present invention has suchan effect that it can be matched to a characteristic impedance of 50ohms (Ω) in any cases of high and low output powers, and hencecharacteristics such as high output power and high efficiency can berealized. In addition, it has effects of preventing the amplified highfrequency signal from passing around the turned-off amplifying elementto the matching circuit, reducing a loss in the output matching circuit,enhancing isolation between the input and the output of the turned-offamplifying element side, and suppressing the oscillation due to thesignal passing around the turned-off amplifying element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating a structure of a high frequencyamplifier according to Embodiment 1 of the present invention.

FIG. 2 is a diagram for illustrating impedance on a Smith chart.

FIGS. 3A-3E are a diagram for illustrating impedance on the Smith chart.

FIGS. 4A-4D are Smith charts illustrating impedance of an outputmatching circuit of the high frequency amplifier according to Embodiment1 of the present invention.

FIG. 5 is a circuit diagram illustrating a structure of a high frequencyamplifier according to Embodiment 2 of the present invention.

FIGS. 6A-6D are Smith charts illustrating impedance of an outputmatching circuit of the high frequency amplifier according to Embodiment2 of the present invention.

FIG. 7 is a circuit diagram illustrating a structure of a high frequencyamplifier according to Embodiment 3 of the present invention.

FIGS. 8A-8D are Smith charts illustrating impedance of an outputmatching circuit of the high frequency amplifier according to Embodiment3 of the present invention.

FIG. 9 is a circuit diagram illustrating a structure of a high frequencyamplifier according to Embodiment 4 of the present invention.

FIGS. 10A-10D are Smith charts illustrating impedance of an outputmatching circuit of the high frequency amplifier according to Embodiment4 of the present invention.

FIG. 11 is a circuit diagram illustrating a structure of a highfrequency amplifier according to Embodiment 5 of the present invention.

FIGS. 12A-12D are Smith charts illustrating impedance of an outputmatching circuit of the high frequency amplifier according to Embodiment5 of the present invention.

FIG. 13 is a circuit diagram illustrating a structure of a highfrequency amplifier according to Embodiment 6 of the present invention.

FIG. 14 is a circuit diagram illustrating a structure of a highfrequency amplifier according to Embodiment 7 of the present invention.

FIG. 15 is a circuit diagram illustrating a structure of a highfrequency amplifier according to Embodiment 8 of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, high frequency amplifiers according to Embodiments 1 to 8of the present invention are described.

Embodiment 1

A high frequency amplifier according to Embodiment 1 of the presentinvention is described with reference to FIGS. 1 to 4. FIG. 1 is acircuit diagram illustrating a structure of the high frequency amplifieraccording to Embodiment 1 of the present invention. Note that the samereference symbols in the individual diagrams denote the same element orequivalent elements in the following description.

In FIG. 1, a high frequency amplifier 100 according to Embodiment 1includes an input terminal 1, an output terminal 2, a collector (drain)bias terminal 4, a base (gate) bias setting terminal 5, and a modeswitching terminal 6.

In addition, the high frequency amplifier 100 includes a high outputpower last phase amplifying element (first amplifying element) 11, a lowoutput power last phase amplifying element (second amplifying element)12, two input matching circuits 13, an output matching circuit 15, andtwo base (gate) bias control circuits (first and second bias controlcircuits) 16. Note that each of the two base (gate) bias controlcircuits 16 is connected to a power source terminal 28.

The output matching circuit 15 includes a first matching circuit 34, asecond matching circuit 35, and a third matching circuit 36. Note thatthe first and second matching circuits 34 and 35 are connected to thethird matching circuit 36 via a connection node 29.

The first matching circuit 34 includes a high pass filter type matchingcircuit (first high pass filter type matching circuit) 27 and a serialinductor 25. In addition, the high pass filter type matching circuit 27includes a short stub made up of a collector (drain) bias line 23 and abypass capacitor 24, a serial capacitor 17, and a parallel inductor 18.Note that an end of each of the bypass capacitor 24 and the parallelinductor 18 is connected to a ground 19.

The second matching circuit 35 includes a high pass filter type matchingcircuit (second high pass filter type matching circuit) 27. In addition,the high pass filter type matching circuit 27 includes a collector(drain) bias applying inductor 26, the bypass capacitor 24, and theserial capacitor 17. Note that an end of the bypass capacitor 24 isconnected to the ground 19.

The third matching circuit 36 includes a low pass filter type matchingcircuit 30. In addition, the low pass filter type matching circuit 30includes the serial inductor 25 and a parallel capacitor 22. Note thatan end of the parallel capacitor 22 is connected to the ground 19.

Here, before describing an operation of the high frequency amplifier,complex impedance (Z=R+jX) [Ω] on a Smith chart is described. FIGS. 2and 3A-3E are diagrams for illustrating impedance on the Smith chart.

In FIG. 2, impedance on a semicircle SCA upper than a line LXO of X=0 isinductive impedance. In other words, if “X>0” holds with respect to“X=jωL>0”, it is inductive impedance. In addition, impedance on asemicircle SCB lower than the line LX0 of X=0 is capacitive impedance.In other words, if “X<0” holds with respect to “X=1/(jωC)=−j/ωC<0”, itis capacitive impedance. The impedance decreases as going to the leftside, and a left end point ZA has impedance Z=0 (R=0, X=0). Theimpedance increases as going to the right side, and a right endpoint ZBhas impedance Z=∞ (infinite value) (R=∞, X=0). Note that though X=±∞precisely when Z=∞, it is regarded that X=0 for convenience sake.Further, a middle point ZC between the left end point ZA and the rightend point ZB has an impedance value Z=50 (ohms) (R=50, X=0). This pointZC is the center of a large circle.

With reference to FIGS. 3A-3E, in the case of a serial capacitor Csillustrated in FIG. 3A, the point of impedance Z_Cs viewed from theright terminal moves in a counterclockwise manner on a circle passingthrough the points of the impedance Z1 and Z=∞ (ZB) as the serialcapacitor Cs decreases as illustrated in FIG. 3E. In the case of aserial inductor Ls illustrated in FIG. 3B, the point of impedance Z_Lsviewed from the right terminal moves in a clockwise manner on a circlepassing through the points of the impedance Z1 and Z=∞ (ZB) as theserial inductor Ls increases as illustrated in FIG. 3E. In the case of aparallel capacitor Cp illustrated in FIG. 3C, the point of impedanceZ_Cp viewed from the right terminal moves in a clockwise manner on acircle passing through the points of the impedance Z1 and Z=0 (ZA) asthe parallel capacitor Cp increases as illustrated in FIG. 3E. In thecase of a parallel inductor Lp illustrated in FIG. 3D, the point ofimpedance Z_Lp viewed from the right terminal moves in acounterclockwise manner on a circle passing through the points of theimpedance Z1 and Z=0 (ZA) as the parallel inductor Lp decreases asillustrated in FIG. 3E.

Next, an operation of the high frequency amplifier according toEmbodiment 1 is described with reference to the drawings. FIGS. 4A-4Dare Smith charts illustrating impedance of the output matching circuitof the high frequency amplifier according to Embodiment 1 of the presentinvention.

The high frequency amplifier 100 includes the high output power lastphase amplifying element 11, the low output power last phase amplifyingelement 12, the two input matching circuits 13, the output matchingcircuit 15, and the two base (gate) bias control circuits 16 asillustrated in FIG. 1. An element size of the high output power lastphase amplifying element 11 is larger than a size of the low outputpower last phase amplifying element 12.

The amplifying elements 11 and 12 are made up of a bipolar transistorsuch as a heterobipolar transistor (HBT) or a bipolar junctiontransistor (BJT), or a field effect transistor (FET) such as ametal-semiconductor FET (MESFET) or a high electron mobility transistor(HEMT).

The high output power last phase amplifying element 11 is supplied witha collector bias voltage that is applied to the collector (drain) biasterminal 4 from the bypass capacitor 24 side via the collector (drain)bias line 23. In addition, the low output power last phase amplifyingelement 12 is supplied with a collector bias voltage that is applied tothe collector (drain) bias terminal 4 from the bypass capacitor 24 sidevia the collector (drain) bias applying inductor 26. Here, the collector(drain) bias applying inductor 26 may be used instead of the collector(drain) bias line 23, and vice versa. Specifically, the collector(drain) bias line 23 may be used instead of the collector (drain) biasapplying inductor 26. In addition, the collector (drain) bias line 23and the collector (drain) bias applying inductor 26 work also asmatching elements.

A base (gate) bias voltage of the amplifying element 11 or 12 issupplied from the voltage that is applied to the base (gate) biassetting terminal 5 via the base (gate) bias control circuit 16. The base(gate) bias control circuit 16 includes a bias circuit for convertingthe voltage applied to the base (gate) bias setting terminal 5 into thebase (gate) voltage to be applied to the amplifying element 11 or 12. Apower source voltage of the base (gate) bias control circuit 16 issupplied from the power source terminal 28.

The base (gate) bias control circuit 16 sets the base (gate) voltage ofthe high output power last phase amplifying element 11 so as to turn onthe high output power last phase amplifying element 11 if the outputpower of the high frequency amplifier 100 is high based on a modeswitching voltage for switching cases of the high output power and thelow output power applied to the mode switching terminal 6. In addition,the base (gate) bias control circuit 16 sets the base (gate) voltage ofthe low output power last phase amplifying element 12 so as to turn offthe low output power last phase amplifying element 12.

On the contrary, the base (gate) bias control circuit 16 sets the base(gate) voltage of the low output power last phase amplifying element 12so as to turn on the low output power last phase amplifying element 12if the output power of the high frequency amplifier 100 is low based onthe mode switching voltage that is applied to the mode switchingterminal 6. In addition, the base (gate) bias control circuit 16 setsthe base (gate) voltage of the high output power last phase amplifyingelement 11 so as to turn off the high output power last phase amplifyingelement 11.

The high frequency signal input from the input terminal 1 is amplifiedby the high output power last phase amplifying element 11 via the inputmatching circuit 13 if the output power of the high frequency amplifier100 is high. After that, it is matched by the first matching circuit 34to medium impedance between impedance of the amplifying element 11 and50 ohms (Ω) that is input and output characteristic impedance of thehigh frequency amplifier 100. After that, it is matched by the thirdmatching circuit 36 to 50 ohms (Ω) that is the characteristic impedanceand is output from the output terminal 2.

The first matching circuit 34 is made up of the high pass filter typematching circuit 27 and the serial inductor 25 as described above. Thehigh pass filter type matching circuit 27 is made up of the short stubincluding the collector (drain) bias line 23 and the bypass capacitor24, the serial capacitor 17, and the parallel inductor 18. In addition,the third matching circuit 36 is made up of the low pass filter typematching circuit 30. The low pass filter type matching circuit 30 ismade up of the serial inductor 25 and the parallel capacitor 22.

Here, the case where the third matching circuit 36 is a single stage ofthe ladder low pass filter type matching circuit 30 is described, but itmay have any circuit structure as long as the matching circuit can matchthe medium impedance to be 50 ohms (Ω). Therefore, it may be amultistage low pass filter type matching circuit, a single stage ormultistage high pass filter type matching circuit, or a matching circuitas a combination of the low pass filter type matching circuit and thehigh pass filter type matching circuit.

If the output power of the high frequency amplifier 100 is low, the highfrequency signal supplied to the input terminal 1 passes through theinput matching circuit 13 and is amplified by the low output power lastphase amplifying element 12. After that, it is matched by the secondmatching circuit 35 to medium impedance between impedance of theamplifying element 12 and 50 ohms (Ω) that is input and outputcharacteristic impedance of the high frequency amplifier 100. Afterthat, it is matched by the third matching circuit 36 to 50 ohms (Ω) thatis characteristic impedance and is output from the output terminal 2.

The second matching circuit 35 is made up of the high pass filter typematching circuit 27 as described above. The high pass filter typematching circuit 27 is made up of the circuit including the collector(drain) bias applying inductor 26 and the bypass capacitor 24, and theserial capacitor 17.

Note that the circuit illustrated in FIG. 1 is made up mainly of lumpedconstant elements, but it is possible that the serial inductor 25 ismade up of serial lines using a distributed constant circuit. Inaddition, the parallel capacitor 22 may be made up of an open stab, andthe parallel inductor 18 may be made up of a short stub.

Here, there is a request for the first matching circuit 34 and thesecond matching circuit 35 concerning the impedance viewed from theconnection node 29 between the first matching circuit 34 and the secondmatching circuit 35. A “first condition” is that the impedance (firstimpedance) of the first matching circuit 34 viewed from the connectionnode 29 in the case where the output power is high, i.e., the case wherethe high output power last phase amplifying element 11 is turned onwhile the low output power last phase amplifying element 12 is turnedoff is substantially the same as the impedance (second impedance) of thesecond matching circuit 35 viewed from the connection node 29 in thecase where the output power is low, i.e., the case where the low outputpower last phase amplifying element 12 is turned on while the highoutput power last phase amplifying element 11 is turned off. Thus, anoutput impedance Zout of the high frequency amplifier 100 can be matchedto 50 ohms (Ω) by the third matching circuit 36 in any case when theamplifying elements 11 and 12 are switched in accordance with a level ofthe output power.

A “second condition” is that the impedance (second impedance) of thesecond matching circuit 35 viewed from the connection node 29 in thecase where the output power is high, i.e., the case where the highoutput power last phase amplifying element 11 is turned on while the lowoutput power last phase amplifying element 12 is turned off issufficiently higher than the impedance (first impedance) of the firstmatching circuit 34 viewed from the connection node 29 in the case wherethe output power is high. Thus, the high frequency signal that isamplified by the high output power last phase amplifying element 11 andthen flows to the connection node 29 via the first matching circuit 34does not pass around to the second matching circuit 35, and it is outputfrom the output terminal 2 through the third matching circuit 36.

Therefore, a loss generated by the high frequency signal that passesaround to the second matching circuit 35 in the output matching circuit15 can be reduced, and hence the characteristics such as the outputpower and the efficiency can be enhanced in the case where the outputpower is high. At the same time, the high frequency signal does not passaround to the second matching circuit 35, and hence the oscillationgenerated by a feedback of the high frequency signal amplified by thehigh output power last phase amplifying element 11 to the input side viathe low output power last phase amplifying element 12 that is turned offcan be suppressed in the case where the output power is high. In otherwords, isolation between the input and the output of the circuit on theside of the low output power last phase amplifying element 12 that isturned off can be enhanced so that the oscillation can be suppressed.

A “third condition” is that the impedance (first impedance) of the firstmatching circuit 34 viewed from the connection node 29 in the case wherethe output power is low, i.e., the case where the low output power lastphase amplifying element 12 is turned on while the high output powerlast phase amplifying element 11 is turned off is sufficiently higherthan the impedance (second impedance) of the second matching circuit 35viewed from the connection node 29 in the case where the output power islow. Thus, the high frequency signal that is amplified by the low outputpower last phase amplifying element 12 and flows to the connection node29 via the second matching circuit 35 does not pass around to the firstmatching circuit 34 but is output from the output terminal 2 via thethird matching circuit 36.

Therefore, the loss generated by the high frequency signal that passesaround to the first matching circuit 34 in the output matching circuit15 can be reduced, and hence the characteristics such as the outputpower and the efficiency in the case where the output power is low canbe enhanced. At the same time, the high frequency signal does not passaround to the first matching circuit 34, and hence the oscillationgenerated by a feedback of the high frequency signal amplified by thelow output power last phase amplifying element 12 to the input side viathe high output power last phase amplifying element 11 that is turnedoff can be suppressed in the case where the output power is low. Inother words, isolation between the input and the output of the circuiton the side of the high output power last phase amplifying element 11that is turned off can be enhanced so that the oscillation can besuppressed.

Here, an operation of the output matching circuit 15 of the highfrequency amplifier 100 illustrated in FIG. 1 is described. FIGS. 4A and4B illustrate loci of impedance of the first matching circuit 34 viewedfrom the connection node 29 in the case where the output power is highand in the case where it is low, respectively, with full line arrows.FIGS. 4C and 4D illustrate loci of impedance of the second matchingcircuit 35 viewed from the connection node 29 in the case where theoutput power is high and in the case where it is low, respectively, withfull line arrows. In addition, a locus of impedance of the circuit fromthe connection node 29 to the output terminal 2 is also illustrated witha dotted line arrow. In FIGS. 4A-4D, Zout1, Z11, Z12, Z13, Z14, Zout2,Z21, Z22, Z3 and Zout respectively denote impedances viewed frompositions indicated on the circuit diagram of FIG. 1.

FIG. 4A is a diagram illustrating impedance of the first matchingcircuit 34 viewed from the connection node 29 in the case where theoutput power is high. FIG. 4B is a diagram illustrating impedance of thefirst matching circuit 34 viewed from the connection node 29 in the casewhere the output power is low. FIG. 4C is a diagram illustratingimpedance of the second matching circuit 35 viewed from the connectionnode 29 in the case where the output power is high. FIG. 4D is a diagramillustrating impedance of the second matching circuit 35 viewed from theconnection node 29 in the case where the output power is low.

With reference to FIGS. 4A and 4D, it is understood that Z14 that isimpedance of the first matching circuit 34 viewed from the connectionnode 29 in the case where the output power is high is substantially thesame as Z22 that is impedance of the second matching circuit 35 viewedfrom the connection node 29 in the case where the output power is low.Therefore, the above-mentioned “first condition” can be satisfied. Thus,the output impedance Zout of the high frequency amplifier 100 can bematched to 50 ohms (Ω) by the third matching circuit 36 in any case whenthe amplifying elements 11 and 12 are switched in accordance with alevel of the output power. Therefore, the high frequency amplifier 100can realize characteristics such as high output power and highefficiency in any cases where the output power is high and where it islow.

With reference to FIGS. 4C and 4A, it is understood that Z22 that isimpedance of the second matching circuit 35 viewed from the connectionnode 29 in the case where the output power is high is sufficientlyhigher than Z14 that is impedance of the first matching circuit 34viewed from the connection node 29 in the case where the output power ishigh. Therefore, the above-mentioned “second condition” can besatisfied. Thus, the loss due to the high frequency signal that passesaround to the second matching circuit 35 in the output matching circuit15 can be reduced, and hence characteristics such as the high outputpower and high efficiency can be realized in the case where the outputpower is high. At the same time, the high frequency signal does not passaround to the second matching circuit 35, and hence the oscillationgenerated by a feedback of the high frequency signal amplified by thehigh output power last phase amplifying element 11 to the input side viathe low output power last phase amplifying element 12 that is turned offcan be suppressed in the case where the output power is high. In otherwords, isolation between the input and the output of the circuit or aside of the low output power last phase amplifying element 12 that isturned off can be enhanced so that the oscillation can be suppressed.

Here, a method of increasing Z22 that is impedance of the secondmatching circuit 35 viewed from the connection node 29 in the case wherethe output power is high is described. The output impedance Zout2 whenthe low output power last phase amplifying element 12 is turned off iscapacitive impedance as illustrated in FIG. 4C, and hence the secondmatching circuit 35 can increase the impedance Z22 viewed from theconnection node 29 by using a high pass filter type matching elementsuch as the collector (drain) bias applying inductor 26 or the serialcapacitor 17. In this way, it is necessary to dispose the high passfilter type matching circuit 27 on the connection node 29 side of thesecond matching circuit 35.

It is understood from FIGS. 4B and 4D that Z14 that is impedance of thefirst matching circuit 34 viewed from the connection node 29 in the casewhere the output power is low is sufficiently higher than Z22 that isimpedance of the second matching circuit 35 viewed from the connectionnode 29 in the case where the output power is low. Therefore, theabove-mentioned “third condition” can be satisfied. Thus, the lossgenerated by the high frequency signal passing around to the firstmatching circuit 34 in the output matching circuit 15 can be reduced,and hence characteristics such as the high output power and highefficiency can be realized in the case where the output power is low. Atthe same time, the high frequency signal does not pass around to thefirst matching circuit 34, and hence the oscillation generated by afeedback of the high frequency signal amplified by the low output powerlast phase amplifying element 12 to the input side via the high outputpower last phase amplifying element 11 that is turned off can besuppressed in the case where the output power is low. In other words,isolation between the input and the output of the circuit on a side ofthe high output power last phase amplifying element 11 that is turnedoff can be enhanced, and hence the oscillation can be suppressed.

Here, a method of increasing Z14 that is impedance of the first matchingcircuit 34 viewed from the connection node 29 in the case where theoutput power is low is described. The output impedance Zout1 when thehigh output power last phase amplifying element 11 is turned off iscapacitive impedance as illustrated in FIG. 4B, and hence the firstmatching circuit 34 can increase the impedance Z14 viewed from theconnection node 29 by providing the short stub made up of the collector(drain) bias line 23 and the bypass capacitor 24, or the high passfilter type matching circuit 27 made up of a high pass filter typematching element such as the serial capacitor 17 and the parallelinductor 18. When the impedance is increased by the high pass filtertype matching circuit 27, it becomes the inductive impedance (Z13).Therefore, the serial inductor 25 is disposed at the position that isclosest to the connection node 29, and hence the impedance is furtherincreased. In this way, it is necessary to dispose the high pass filtertype matching circuit 27 and the serial inductor 25 on the connectionnode 29 side of the first matching circuit 34.

According to Embodiment 1, the high frequency amplifier 100 illustratedin FIG. 1 includes the first matching circuit 34 disposed on the outputside of the high output power last phase amplifying element 11, thesecond matching circuit 35 disposed on the output side of the low outputpower last phase amplifying element 12 and the third matching circuit 36disposed on the post stage thereof. Therefore, it can be matched to thecharacteristic impedance of 50 ohms (Ω) in any cases of high and lowoutput powers, and hence characteristics such as the high output powerand high efficiency can be realized as the high frequency amplifier.

In addition, the impedance of the matching circuit on the turned-offamplifying element side viewed from the connection node 29 can besufficiently higher than the impedance of the matching circuit on theturned-on amplifying element side viewed from the connection node 29 inany cases of high and low output powers. Therefore, it is possible toprevent the amplified high frequency signal from passing around to thematching circuit on the turned-off amplifying element side, and hence aloss in the output matching circuit 15 can be reduced, wherebycharacteristics such as the high output power and high efficiency can berealized as the high frequency amplifier. Further, isolation between theinput and the output on the turned-off amplifying element side can beenhanced, and the oscillation due to the signal passing around theturned-off amplifying element can be suppressed.

Embodiment 2

A high frequency amplifier according to Embodiment 2 of the presentinvention is described with reference to FIGS. 5 and 6A-6D. FIG. 5 is acircuit diagram illustrating a structure of the high frequency amplifieraccording to Embodiment 2 of the present invention.

In FIG. 5, a high frequency amplifier 100 according to Embodiment 2includes an input terminal 1, an output terminal 2, a collector (drain)bias terminal 4, a base (gate) bias setting terminal 5, and a modeswitching terminal 6.

In addition, the high frequency amplifier 100 includes a high outputpower last phase amplifying element (first amplifying element) 11, a lowoutput power last phase amplifying element (second amplifying element)12, two input matching circuits 13, an output matching circuit 15, andtwo base (gate) bias control circuits (first and second bias controlcircuits) 16. An element size of the high output power last phaseamplifying element 11 is larger than a size of the low output power lastphase amplifying element 12. Note that each of the two base (gate) biascontrol circuits 16 is connected to a power source terminal 28.

The output matching circuit 15 includes a first matching circuit 34, asecond matching circuit 35, and a third matching circuit 36. Note thatthe first and second matching circuits 34 and 35 are connected to thethird matching circuit 36 via a connection node 29.

The first matching circuit 34 includes a high pass filter type matchingcircuit (first high pass filter type matching circuit) 27. In addition,the high pass filter type matching circuit 27 includes a short stub madeup of a collector (drain) bias line 23 and a bypass capacitor 24. Notethat an end of the bypass capacitor 24 is connected to a ground 19.

The second matching circuit 35 includes a serial inductor 25 and a highpass filter type matching circuit (second high pass filter type matchingcircuit) 27. In addition, the high pass filter type matching circuit 27includes a collector (drain) bias applying inductor 26, the bypasscapacitor 24, and a serial capacitor 17. Note that an end of the bypasscapacitor 24 is connected to the ground 19.

The third matching circuit 36 includes a low pass filter type matchingcircuit 30. In addition, the low pass filter type matching circuit 30includes two stages of circuits made up of the serial inductor 25 and aparallel capacitor 22. Note that an end of each of the two parallelcapacitors 22 is connected to the ground 19.

The high frequency amplifier 100 of Embodiment 2 illustrated in FIG. 5is different from the high frequency amplifier 100 of Embodiment 1illustrated in FIG. 1 in that the first matching circuit 34 is made upof only the high pass filter type matching circuit 27 and that the highpass filter type matching circuit 27 is made up of only the short stubincluding the collector (drain) bias line 23 and the bypass capacitor24.

In addition, it is also different in that the second matching circuit 35is made up of the serial inductor 25 and the high pass filter typematching circuit 27.

Further, it is different in that the third matching circuit 36 is madeup of the low pass filter type matching circuit 30 having two stages.

Next, an operation of the high frequency amplifier according toEmbodiment 2 is described with reference to the drawings. FIGS. 6A-6Dare Smith charts illustrating impedance of the output matching circuitof the high frequency amplifier according to Embodiment 2 of the presentinvention.

A flow of the signal in the high frequency amplifier 100, a method ofapplying the bias, and the conditions required to the output matchingcircuit 15 are the same as those in the above-mentioned Embodiment 1,and hence descriptions thereof are omitted.

An operation of the output matching circuit 15 of the high frequencyamplifier 100 illustrated in FIG. 5 is described. FIGS. 6A and 6Billustrate loci of impedance of the first matching circuit 34 viewedfrom the connection node 29 in the case where the output power is highand in the case where it is low, respectively, with full line arrows.FIGS. 6C and 6D illustrate loci of impedance of the second matchingcircuit 35 viewed from the connection node 29 in the case where theoutput power is high and in the case where it is low, respectively, withfull line arrows. In addition, a locus of impedance of the circuit fromthe connection node 29 to the output terminal 2 is also illustrated witha dotted line arrow. In FIGS. 6A-6D, Zout1, Z11, Zout2, Z21, Z22, Z23,Z3, Z4, Z5, and Zout respectively denote impedances viewed frompositions indicated on the circuit diagram of FIG. 5.

FIG. 6A is a diagram illustrating impedance of the first matchingcircuit 34 viewed from the connection node 29 in the case where theoutput power is high. FIG. 6B is a diagram illustrating impedance of thefirst matching circuit 34 viewed from the connection node 29 in the casewhere the output power is low. FIG. 6C is a diagram illustratingimpedance of the second matching circuit 35 viewed from the connectionnode 29 in the case where the output power is high. FIG. 6D is a diagramillustrating impedance of the second matching circuit 35 viewed from theconnection node 29 in the case where the output power is low.

With reference to FIGS. 6A and 6D, it is understood that Z11 that isimpedance (first impedance) of the first matching circuit 34 viewed fromthe connection node 29 in the case where the output power is high issubstantially the same as Z23 that is impedance (second impedance) ofthe second matching circuit 35 viewed from the connection node 29 in thecase where the output power is low. Therefore, the above-mentioned“first condition” can be satisfied. Thus, the output impedance Zout ofthe high frequency amplifier 100 can be matched to 50 ohms (Ω) by thethird matching circuit 36 in any case when the amplifying elements 11and 12 are switched in accordance with a level of the output power.Therefore, the high frequency amplifier 100 can realize characteristicssuch as high output power and high efficiency in any cases where theoutput power is high and where it is low.

With reference to FIGS. 6C and 6A, it is understood that Z23 that isimpedance (second impedance) of the second matching circuit 35 in thecase where the output power is high is sufficiently higher than Z11 thatis impedance (first impedance) of the first matching circuit 34 viewedfrom the connection node 29 in the case where the output power is high.Therefore, the above-mentioned “second condition” can be satisfied.Thus, the loss due to the high frequency signal that passes around tothe second matching circuit 35 in the output matching circuit 15 can bereduced, and hence characteristics such as the high output power andhigh efficiency can be realized in the case where the output power ishigh. At the same time, the high frequency signal does not pass aroundto the second matching circuit 35, and hence the oscillation generatedby a feedback of the high frequency signal amplified by the high outputpower last phase amplifying element 11 to the input side via the lowoutput power last phase amplifying element 12 that is turned off can besuppressed in the case where the output power is high. In other words,isolation between the input and the output of the circuit on a side ofthe low output power last phase amplifying element 12 that is turned offcan be enhanced so that the oscillation can be suppressed.

Here, a method of increasing Z23 that is impedance of the secondmatching circuit 35 in the case where the output power is high isdescribed. The output impedance Zout2 when the low output power lastphase amplifying element 12 is turned off is capacitive impedance asillustrated in FIG. 6C, and hence, though the second matching circuit 35is connected to the serial inductor 25 on the output side of theamplifying element 12, the size thereof should fall on a range in whichthe impedance is capacitive, which should not be too large. In view ofthis, the second matching circuit 35 can increase the impedance Z23viewed from the connection node 29 by using a high pass filter typematching element such as the collector (drain) bias applying inductor 26or the serial capacitor 17. In this way, it is necessary to dispose theserial inductor 25 and the high pass filter type matching circuit 27 onthe connection node 29 side of the second matching circuit 35.

It is understood from FIGS. 6B and 6D that Z11 that is impedance (firstimpedance) of the first matching circuit 34 viewed from the connectionnode 29 in the case where the output power is low is sufficiently higherthan Z23 that is impedance (second impedance) of the second matchingcircuit 35 viewed from the connection node 29 in the case where theoutput power is low. Therefore, the above-mentioned “third condition”can be satisfied. Thus, the loss generated by the high frequency signalpassing around to the first matching circuit 34 in the output matchingcircuit 15 can be reduced, and hence characteristics such as the highoutput power and high efficiency can be realized in the case where theoutput power is low. At the same time, the high frequency signal doesnot pass around to the first matching circuit 34, and hence theoscillation generated by a feedback of the high frequency signalamplified by the low output power last phase amplifying element 12 tothe input side via the high output power last phase amplifying element11 that is turned off can be suppressed in the case where the outputpower is low. In other words, isolation between the input and the outputof the circuit on a side of the high output power last phase amplifyingelement 11 that is turned off can be enhanced, and hence the oscillationcan be suppressed.

Here, a method of increasing Z11 that is impedance of the first matchingcircuit 34 in the case where the output power is low is described. Theoutput impedance Zout1 when the high output power last phase amplifyingelement 11 is turned off is capacitive impedance as illustrated in FIG.6B, and hence the first matching circuit 34 can increase the impedanceZ11 viewed from the connection node 29 by providing the high pass filtertype matching circuit 27 made up of the short stub. The short stub ismade up of the collector (drain) bias line 23 and the bypass capacitor24. In other words, the first matching circuit 34 includes the high passfilter type matching circuit 27 disposed at the position that is closestto the connection node 29.

In addition, the high frequency amplifier 100 according to Embodiment 2is different from the high frequency amplifier 100 according to theabove-mentioned Embodiment 1 as follows. The matching circuit betweenthe high output power last phase amplifying element 11 that is turned onin the case where the output power is high and the output terminal 2 ismade up of the high pass filter type matching circuit that works also asa bias circuit partially and a low pass filter type matching circuit inEmbodiment 1. In contrast, all the circuits except the bias circuit aremade up of the low pass filter type matching circuit in Embodiment 2.

The high pass filter type matching circuit has a problem that if aparallel inductor is used in a low impedance, a loss due to a parasiticresistance of the inductor becomes large. In the high frequencyamplifier 100 according to Embodiment 2, the output matching circuit 15is made up mainly of the low pass filter type matching circuit, andhence a loss in the output matching circuit 15 in the case where theoutput power is high is reduced, whereby the high frequency amplifier100 can have higher output power as well as higher efficiency comparedwith the high frequency amplifier 100 according to Embodiment 1.

According to Embodiment 2, the high frequency amplifier 100 illustratedin FIG. 5 includes the first matching circuit 34 disposed on the outputside of the high output power last phase amplifying element 11, thesecond matching circuit 35 disposed on the output side of the low outputpower last phase amplifying element 12 and the third matching circuit 36disposed on the post stage thereof. Therefore, it can be matched to thecharacteristic impedance of 50 ohms (Ω) in any cases of high and lowoutput powers, and hence characteristics such as the high output powerand high efficiency can be realized as the high frequency amplifier.

In addition, the impedance of the matching circuit on the turned-offamplifying element side viewed from the connection node 29 can besufficiently higher than the impedance of the matching circuit on theturned-on amplifying element side viewed from the connection node 29 inany cases of high and low output powers. Therefore, it is possible toprevent the amplified high frequency signal from passing around to thematching circuit on the turned-off amplifying element side, and hence aloss in the output matching circuit 15 can be reduced, wherebycharacteristics such as the high output power and high efficiency can berealized as the high frequency amplifier. Further, isolation between theinput and the output on the turned-off amplifying element side can beenhanced, and the oscillation due to the signal passing around theturned-off amplifying element can be suppressed.

Further, in the case where the output power is high, the output matchingcircuit 15 when the high output power last phase amplifying element 11is turned on is made up mainly of the low pass filter type matchingcircuit, and hence a loss in the output matching circuit 15 in the casewhere the output power is high can be reduced, whereby the highfrequency amplifier 100 can have higher output power as well as higherefficiency.

Note that the circuit described in Embodiment 2 is made up mainly oflumped constant elements, but the serial inductor 25 may be made up of aserial line, the parallel capacitor 22 may be made up of an open stub,and the parallel inductor may be made up of a short stub. The amplifyingelements 11 and 12 are made up of a heterobipolar transistor (HBT), butmay be made up of another bipolar transistor or a field effecttransistor (FET) such as a metal-semiconductor FET (MESFET) or a highelectron mobility transistor (HEMT). Further, the collector (drain) biasapplying inductor 26 may be used instead of the collector (drain) biasline 23, and vice versa. Specifically, the collector (drain) bias line23 may be used instead of the collector (drain) bias applying inductor26. In addition, the collector (drain) bias line 23 and the collector(drain) bias applying inductor 26 work also as matching elements.

Embodiment 3

A high frequency amplifier according to Embodiment 3 of the presentinvention is described with reference to FIGS. 7 and 8A-8D. FIG. 7 is acircuit diagram illustrating a structure of the high frequency amplifieraccording to Embodiment 3 of the present invention.

In FIG. 7, a high frequency amplifier 100 according to Embodiment 3includes an input terminal 1, an output terminal 2, a collector (drain)bias terminal 4, a base (gate) bias setting terminal 5, and a modeswitching terminal 6.

In addition, the high frequency amplifier 100 includes a high outputpower last phase amplifying element (first amplifying element) 11, a lowoutput power last phase amplifying element (second amplifying element)12, two input matching circuits 13, an output matching circuit 15, andtwo base (gate) bias control circuits (first and second bias controlcircuits) 16. An element size of the high output power last phaseamplifying element 11 is larger than a size of the low output power lastphase amplifying element 12. Note that each of the two base (gate) biascontrol circuits 16 is connected to a power source terminal 28.

The output matching circuit 15 includes a first matching circuit 34, asecond matching circuit 35, and a third matching circuit 36. Note thatthe first and second matching circuits 34 and 35 are connected to thethird matching circuit 36 via a connection node 29.

The first matching circuit 34 includes a short stub made up of acollector (drain) bias line 23 and a bypass capacitor 24, a low passfilter type matching circuit 30, and a serial inductor (first serialinductor) 25. In addition, the low pass filter type matching circuit 30includes two stages of circuits made up of a serial inductor (thirdserial inductor) 25 and a parallel capacitor (first parallel capacitor)22. Note that an end of each of the bypass capacitor 24 and parallelcapacitor 22 is connected to a ground 19.

The second matching circuit 35 includes a high pass filter type matchingcircuit 27 and a serial inductor (second serial inductor) 25. Inaddition, the high pass filter type matching circuit 27 includes acollector (drain) bias applying inductor 26, the bypass capacitor 24,and a serial capacitor 17. Note that an end of the bypass capacitor 24is connected to the ground 19.

The third matching circuit 36 includes the serial capacitor 17.

The high frequency amplifier 100 of Embodiment 3 illustrated in FIG. 7is different from the high frequency amplifier 100 of Embodiment 1illustrated in FIG. 1 in that the first matching circuit 34 is made upof the short stub including the collector (drain) bias line 23 and thebypass capacitor 24, the low pass filter type matching circuit 30including two stages of circuits including the serial inductor 25 andthe parallel capacitor 22, and the serial inductor 25.

In addition, it is different in that the second matching circuit 35 ismade up of the circuit including the collector (drain) bias applyinginductor 26 and the bypass capacitor 24, the high pass filter typematching circuit 27 including the serial capacitor 17, and the serialinductor 25.

Further, it is different in that the third matching circuit 36 is madeup of only the serial capacitor 17. However, the third matching circuit36 is made up of only the serial capacitor 17 in FIG. 7, but it may haveany circuit structure as long as it is a matching circuit that can matchthe medium impedance to 50 ohms (Ω). The third matching circuit 36 maybe made up of a circuit including a serial capacitor and a serialinductor. In addition, similarly to the above-mentioned Embodiment 1, itmay be made up of a single stage of the low pass filter type matchingcircuit 30, a multistage low pass filter type matching circuit, a singlestage or multistage high pass filter type matching circuit, or amatching circuit that is a combination of the low pass filter typematching circuit and the high pass filter type matching circuit.

Next, an operation of the high frequency amplifier according toEmbodiment 3 is described with reference to the drawings. FIGS. 8A-8Dare Smith charts illustrating impedance of the output matching circuitof the high frequency amplifier according to Embodiment 3 of the presentinvention.

A flow of the signal in the high frequency amplifier 100, a method ofapplying the bias, and the conditions required to the output matchingcircuit 15 are the same as those in the above-mentioned embodiments, andhence descriptions thereof are omitted.

An operation of the output matching circuit 15 of the high frequencyamplifier 100 illustrated in FIG. 7 is described. FIGS. 8A and 8Billustrate loci of impedance of the first matching circuit 34 viewedfrom the connection node 29 in the case where the output power is highand in the case where it is low, respectively, with full line arrows.FIGS. 8C and 8D illustrate loci of impedance of the second matchingcircuit 35 viewed from the connection node 29 in the case where theoutput power is high and in the case where it is low, respectively, withfull line arrows. In addition, a locus of impedance of the circuit fromthe connection node 29 to the output terminal 2 is also illustrated witha dotted line arrow. In FIGS. 8A-8D, Zout1, Z11, Z12, Z13, Z14, Z15,Z16, Zout2, Z21, Z22, Z23 and Zout respectively denote impedances viewedfrom positions indicated on the circuit diagram of FIG. 7.

FIG. 8A is a diagram illustrating impedance of the first matchingcircuit 34 viewed from the connection node 29 in the case where theoutput power is high. FIG. 8B is a diagram illustrating impedance of thefirst matching circuit 34 viewed from the connection node 29 in the casewhere the output power is low. FIG. 8C is a diagram illustratingimpedance of the second matching circuit 35 viewed from the connectionnode 29 in the case where the output power is high. FIG. 8D is a diagramillustrating impedance of the second matching circuit 35 viewed from theconnection node 29 in the case where the output power is low.

With reference to FIGS. 8A and 8D, it is understood that Z16 that isimpedance (first impedance) of the first matching circuit 34 viewed fromthe connection node 29 in the case where the output power is high issubstantially the same as Z23 that is impedance (second impedance) ofthe second matching circuit 35 viewed from the connection node 29 in thecase where the output power is low. Therefore, the above-mentioned“first condition” can be satisfied. Thus, the output impedance Zout ofthe high frequency amplifier 100 can be matched to 50 ohms (Ω) by thethird matching circuit 36 in any case when the amplifying elements 11and 12 are switched in accordance with a level of the output power.Therefore, the high frequency amplifier 100 can realize characteristicssuch as high output power and high efficiency in any cases where theoutput power is high and where it is low. In addition, the impedancesZ16 and Z23 are matched to substantially 50 ohms (Ω), and hence a simplecircuit structure made up of only the serial capacitor 17 is sufficientfor matching to 50 ohms (Ω).

With reference to FIGS. 8C and 8A, it is understood that Z23 that isimpedance (second impedance) of the second matching circuit 35 viewedfrom the connection node 29 in the case where the output power is highis sufficiently higher than Z16 that is impedance (first impedance) ofthe first matching circuit 34 viewed from the connection node 29 in thecase where the output power is high. Therefore, the above-mentioned“second condition” can be satisfied. Thus, the loss due to the highfrequency signal that passes around to the second matching circuit 35 inthe output matching circuit 15 can be reduced, and hence characteristicssuch as the high output power and high efficiency can be realized in thecase where the output power is high. At the same time, the highfrequency signal does not pass around to the second matching circuit 35,and hence the oscillation generated by a feedback of the high frequencysignal amplified by the high output power last phase amplifying element11 to the input side via the low output power last phase amplifyingelement 12 that is turned off can be suppressed in the case where theoutput power is high. In other words, isolation between the input andthe output of the circuit on a side of the low output power last phaseamplifying element 12 that is turned off can be enhanced so that theoscillation can be suppressed.

Here, a method of increasing Z23 that is impedance of the secondmatching circuit 35 viewed from the connection node 29 in the case wherethe output power is high is described. The output impedance Zout2 whenthe low output power last phase amplifying element 12 is turned off iscapacitive impedance as illustrated in FIG. 8C, and hence the secondmatching circuit 35 can increase the impedance Z23 viewed from theconnection node 29 by using a high pass filter type matching elementsuch as the collector (drain) bias applying inductor 26 or the serialcapacitor 17. In view of this, the second matching circuit 35 includesthe serial inductor 25 at a position that is closest to the connectionnode 29 from the fact that the impedance Z22 is capacitive impedance. Inthis way, it is necessary to dispose the high pass filter type matchingcircuit 27 and the serial inductor 25 on the connection node 29 side ofthe second matching circuit 35.

It is understood from FIGS. 8B and 8D that Z16 that is impedance (firstimpedance) of the first matching circuit 34 viewed from the connectionnode 29 in the case where the output power is low is sufficiently higherthan Z23 that is impedance (second impedance) of the second matchingcircuit 35 viewed from the connection node 29 in the case where theoutput power is low. Therefore, the above-mentioned “third condition”can be satisfied. Thus, the loss generated by the high frequency signalpassing around to the first matching circuit 34 in the output matchingcircuit 15 can be reduced, and hence characteristics such as the highoutput power and high efficiency can be realized in the case where theoutput power is low. At the same time, the high frequency signal doesnot pass around to the first matching circuit 34, and hence theoscillation generated by a feedback of the high frequency signalamplified by the low output power last phase amplifying element 12 tothe input side via the high output power last phase amplifying element11 that is turned off can be suppressed in the case where the outputpower is low. In other words, isolation between the input and the outputof the circuit on a side of the high output power last phase amplifyingelement 11 that is turned off can be enhanced, and hence the oscillationcan be suppressed.

Here, a method of increasing the impedance Z16 of the first matchingcircuit 34 viewed from the connection node 29 in the case where theoutput power is low is described. The output impedance Zout1 when thehigh output power last phase amplifying element 11 is turned off iscapacitive impedance as illustrated in FIG. 8B, and hence the firstmatching circuit 34 can increase the impedance Z11 viewed from theconnection node 29 by providing a short stub. This short stub is made upof the collector (drain) bias line 23 and the bypass capacitor 24.However, for a purpose of matching when the high output power last phaseamplifying element 11 is turned on, the first matching circuit 34includes the low pass filter type matching circuit 30. Therefore, theimpedance is decreased (Z12→Z13→Z14→Z15). In this case, the impedanceZ15 is inductive impedance, and hence it is necessary to insert theserial inductor 25 at the position that is the closest to the connectionnode 29 so as to increase the impedance (Z16). In this way, it isnecessary to dispose the serial inductor 25 on the connection node 29side in the first matching circuit 34.

In addition, the high frequency amplifier 100 according to Embodiment 3is different from the high frequency amplifier 100 according to theabove-mentioned Embodiment 1 as follows. The matching circuit betweenthe high output power last phase amplifying element 11 that is turned onin the case where the output power is high and the output terminal 2 ismade up of the high pass filter type matching circuit that works also asa bias circuit partially and a low pass filter type matching circuit inEmbodiment 1. In contrast, the circuit except the bias circuit is madeup mainly of the low pass filter type matching circuit in Embodiment 3.

The high pass filter type matching circuit has a problem that if aparallel inductor is used in a low impedance, a loss due to a parasiticresistance of the inductor becomes large. In the high frequencyamplifier 100 according to Embodiment 3, the output matching circuit 15is made up mainly of the low pass filter type matching circuit, andhence a loss in the output matching circuit 15 in the case where theoutput power is high is reduced, whereby the high frequency amplifier100 can have higher output power as well as higher efficiency comparedwith the high frequency amplifier 100 according to Embodiment 1.

According to Embodiment 3, the high frequency amplifier 100 illustratedin FIG. 7 includes the first matching circuit 34 disposed on the outputside of the high output power last phase amplifying element 11, thesecond matching circuit 35 disposed on the output side of the low outputpower last phase amplifying element 12 and the third matching circuit 36disposed on the post stage thereof. Therefore, it can be matched to thecharacteristic impedance of 50 ohms (Ω) in any cases of high and lowoutput powers, and hence characteristics such as the high output powerand high efficiency can be realized as the high frequency amplifier.

In addition, the impedance of the matching circuit on the turned-offamplifying element side viewed from the connection node 29 can besufficiently higher than the impedance of the matching circuit on theturned-on amplifying element side viewed from the connection node 29 inany cases of high and low output powers. Therefore, it is possible toprevent the amplified high frequency signal from passing around to thematching circuit on the turned-off amplifying element side, and hence aloss in the output matching circuit 15 can be reduced, wherebycharacteristics such as the high output power and high efficiency can berealized as the high frequency amplifier. Further, isolation between theinput and the output on the turned-off amplifying element side can beenhanced, and the oscillation due to the signal passing around theturned-off amplifying element can be suppressed.

Further, in the case where the output power is high, the output matchingcircuit 15 when the high output power last phase amplifying element 11is turned on is made up mainly of the low pass filter type matchingcircuit, and hence a loss in the output matching circuit 15 in the casewhere the output power is high can be reduced, whereby the highfrequency amplifier 100 can have higher output power as well as higherefficiency.

Note that the circuit described in Embodiment 3 is made up mainly oflumped constant elements, but the serial inductor 25 may be made up of aserial line, the parallel capacitor 22 may be made up of an open stub,and the parallel inductor may be made up of a short stub. The amplifyingelements 11 and 12 are made up of a heterobipolar transistor (HBT), butmay be made up of another bipolar transistor or a field effecttransistor (FET) such as a metal-semiconductor FET (MESFET) or a highelectron mobility transistor (HEMT). Further, the collector (drain) biasapplying inductor 26 may be used instead of the collector (drain) biasline 23, and vice versa. Specifically, the collector (drain) bias line23 may be used instead of the collector (drain) bias applying inductor26. In addition, the collector (drain) bias line 23 and the collector(drain) bias applying inductor 26 work also as matching elements.

Embodiment 4

A high frequency amplifier according to Embodiment 4 of the presentinvention is described with reference to FIGS. 9 and 10A-10D. FIG. 9 isa circuit diagram illustrating a structure of the high frequencyamplifier according to Embodiment 4 of the present invention.

In FIG. 9, a high frequency amplifier 100 according to Embodiment 4includes an input terminal 1, an output terminal 2, a collector (drain)bias terminal 4, a base (gate) bias setting terminal 5, and a modeswitching terminal 6.

In addition, the high frequency amplifier 100 includes a high outputpower last phase amplifying element (first amplifying element) 11, a lowoutput power last phase amplifying element (second amplifying element)12, two input matching circuits 13, an output matching circuit 15, andtwo base (gate) bias control circuits (first and second bias controlcircuits) 16. An element size of the high output power last phaseamplifying element 11 is larger than a size of the low output power lastphase amplifying element 12. Note that each of the two base (gate) biascontrol circuits 16 is connected to a power source terminal 28.

The output matching circuit 15 includes a first matching circuit 34, asecond matching circuit 35, and a third matching circuit 36. Note thatthe first and second matching circuits 34 and 35 are connected to thethird matching circuit 36 via a connection node 29.

The first matching circuit 34 includes a short stub made up of acollector (drain) bias line 23 and a bypass capacitor 24, a low passfilter type matching circuit 30 and a serial inductor (first serialinductor) 25. In addition, the low pass filter type matching circuit 30includes two stages of circuits made up of a parallel capacitor (secondparallel capacitor) 22, a serial inductor (third serial inductor) 25,and a parallel capacitor (first parallel capacitor) 22. Note that an endof each of the bypass capacitor 24 and the parallel capacitor 22 isconnected to a ground 19.

The second matching circuit 35 includes a high pass filter type matchingcircuit 27 and a serial inductor (second serial inductor) 25. Inaddition, the high pass filter type matching circuit 27 includes acollector (drain) bias applying inductor 26, the bypass capacitor 24,and a serial capacitor 17. Note that an end of the bypass capacitor 24is connected to the ground 19.

The third matching circuit 36 includes the serial capacitor 17.

The high frequency amplifier 100 of Embodiment 4 illustrated in FIG. 9is different from the high frequency amplifier 100 of Embodiment 3illustrated in FIG. 7 only in that the parallel capacitor 22 is added tothe position that is the closest to the amplifying element 11 in the lowpass filter type matching circuit 30 of the first matching circuit 34.

Next, an operation of the high frequency amplifier according toEmbodiment 4 is described with reference to the drawings. FIGS. 10A-10Dare Smith charts illustrating impedance of the output matching circuitof the high frequency amplifier according to Embodiment 4 of the presentinvention.

A flow of the signal in the high frequency amplifier 100, a method ofapplying the bias, and the conditions required to the output matchingcircuit 15 are the same as those in the above-mentioned embodiments, andhence descriptions thereof are omitted.

An operation of the output matching circuit 15 of the high frequencyamplifier 100 illustrated in FIG. 9 is described. FIGS. 10A and 10Billustrate loci of impedance of the first matching circuit 34 viewedfrom the connection node 29 in the case where the output power is highand in the case where it is low, respectively, with full line arrows.FIGS. 10C and 10D illustrate loci of impedance of the second matchingcircuit 35 viewed from the connection node 29 in the case where theoutput power is high and in the case where it is low, respectively, withfull line arrows. In addition, a locus of impedance of the circuit fromthe connection node 29 to the output terminal 2 is also illustrated witha dotted line arrow. In FIGS. 10A-10D, Zout1, Z11, Z12, Z13, Z14, Z15,Z16, Z17, Zout2, Z21, Z22, Z23 and Zout respectively denote impedancesviewed from positions indicated on the circuit diagram of FIG. 9.

FIG. 10A is a diagram illustrating impedance of the first matchingcircuit 34 viewed from the connection node 29 in the case where theoutput power is high. FIG. 10B is a diagram illustrating impedance ofthe first matching circuit 34 viewed from the connection node 29 in thecase where the output power is low. FIG. 10C is a diagram illustratingimpedance of the second matching circuit 35 viewed from the connectionnode 29 in the case where the output power is high. FIG. 10D is adiagram illustrating impedance of the second matching circuit 35 viewedfrom the connection node 29 in the case where the output power is low.

With reference to FIGS. 10A and 10D, it is understood that Z17 that isimpedance (first impedance) of the first matching circuit 34 viewed fromthe connection node 29 in the case where the output power is high issubstantially the same as Z23 that is impedance (second impedance) ofthe second matching circuit 35 viewed from the connection node 29 in thecase where the output power is low. In addition, the impedances Z17 andZ23 are matched to substantially 50 ohms (Ω). The locus of the impedanceof the parallel capacitor 22 that is newly added to the position closestto the high output power last phase amplifying element 11 in the lowpass filter type matching circuit 30 is small from Z11 to Z12 becausethe impedance value is small. Therefore, it is understood that the newlyadded parallel capacitor 22 does not cause a problem for the matching.Therefore, the above-mentioned “first condition” can be satisfied. Thus,the output impedance Zout of the high frequency amplifier 100 can bematched to 50 ohms (Ω) by the third matching circuit 36 in any case whenthe amplifying elements 11 and 12 are switched in accordance with alevel of the output power. Therefore, the high frequency amplifier 100can realize characteristics such as high output power and highefficiency in any cases where the output power is high and where it islow.

With reference to FIGS. 10C and 10A, it is understood that Z23 that isimpedance (second impedance) of the second matching circuit 35 in thecase where the output power is high is sufficiently higher than Z17 thatis impedance (first impedance) of the first matching circuit 34 viewedfrom the connection node 29 in the case where the output power is high.Therefore, the above-mentioned “second condition” can be satisfied.Thus, the loss due to the high frequency signal that passes around tothe second matching circuit 35 in the output matching circuit 15 can bereduced, and hence characteristics such as the high output power andhigh efficiency can be realized in the case where the output power ishigh. At the same time, the high frequency signal does not pass aroundto the second matching circuit 35, and hence the oscillation generatedby a feedback of the high frequency signal amplified by the high outputpower last phase amplifying element 11 to the input side via the lowoutput power last phase amplifying element 12 that is turned off can besuppressed in the case where the output power is high. In other words,isolation between the input and the output of the circuit on a side ofthe low output power last phase amplifying element 12 that is turned offcan be enhanced so that the oscillation can be suppressed.

Here, a method of increasing Z23 that is impedance of the secondmatching circuit 35 viewed from the connection node 29 in the case wherethe output power is high is described. The output impedance Zout2 whenthe low output power last phase amplifying element 12 is turned off iscapacitive impedance as illustrated in FIG. 10C, and hence the secondmatching circuit 35 can increase the impedance viewed from theconnection node 29 by using a high pass filter type matching elementsuch as the collector (drain) bias applying inductor 26 or the serialcapacitor 17. In view of this, the serial inductor 25 is provided at aposition that is closest to the connection node 29 from the fact thatthe impedance Z22 is capacitive. In this way, it is necessary to disposethe high pass filter type matching circuit 27 and the serial inductor 25on the connection node 29 side of the second matching circuit 35.

It is understood from FIGS. 10B and 10D that Z17 that is impedance(first impedance) of the first matching circuit 34 viewed from theconnection node 29 in the case where the output power is low issufficiently higher than Z23 that is impedance (second impedance) of thesecond matching circuit 35 viewed from the connection node 29 in thecase where the output power is low. The newly added parallel capacitor22 is inserted at the position that is closest to the high output powerlast phase amplifying element 11 in the low pass filter type matchingcircuit 30, and hence the impedance Z17 of the first matching circuit 34viewed from the connection node 29 can be increased compared with thecase where it is not inserted as illustrated in FIG. 8B. Therefore, theabove-mentioned “third condition” can be satisfied. Thus, the lossgenerated by the high frequency signal passing around to the firstmatching circuit 34 in the output matching circuit 15 can be reduced,and hence characteristics such as the high output power and highefficiency can be realized in the case where the output power is low. Atthe same time, the high frequency signal does not pass around to thefirst matching circuit 34, and hence the oscillation generated by afeedback of the high frequency signal amplified by the low output powerlast phase amplifying element 12 to the input side via the high outputpower last phase amplifying element 11 that is turned off can besuppressed in the case where the output power is low. In other words,isolation between the input and the output of the circuit on a side ofthe high output power last phase amplifying element 11 that is turnedoff can be enhanced, and hence the oscillation can be suppressed.

Here, a method of increasing the impedance Z17 of the first matchingcircuit 34 viewed from the connection node 29 in the case where theoutput power is low is described. The output impedance Zout1 when thehigh output power last phase amplifying element 11 is turned off iscapacitive impedance as illustrated in FIG. 10B, and hence the firstmatching circuit 34 can increase the impedance by providing a shortstub. This short stub is made up of the collector (drain) bias line 23and the bypass capacitor 24. However, for a purpose of matching when thehigh output power last phase amplifying element 11 is turned on, the lowpass filter type matching circuit 30 is provided. Therefore, theimpedance is decreased. In this case, the impedance (Z16) is inductive,and hence it is necessary to insert the serial inductor 25 at theposition that is the closest to the connection node 29 so as to increasethe impedance.

In addition, comparing the high frequency amplifier 100 according toEmbodiment 4 with the high frequency amplifier 100 according to theabove-mentioned Embodiment 3, the impedance Z17 of the first matchingcircuit 34 viewed from the connection node 29 can be increased more bynewly inserting the parallel capacitor 22 at the position that isclosest to the amplifying element 11 in the low pass filter typematching circuit 30 in the case where the output power is low, asdescribed above. Therefore, a loss generated by the high frequencysignal passing around to the first matching circuit 34 in the outputmatching circuit 15 can be further reduced in the case where the outputpower is low, and hence the output power and the efficiency can befurther enhanced in the case where the output power is low. At the sametime, the high frequency signal does not pass around to the firstmatching circuit 34, and hence the oscillation generated by a feedbackof the high frequency signal amplified by the low output power lastphase amplifying element 12 to the input side via the high output powerlast phase amplifying element 11 that is turned off can be furthersuppressed in the case where the output power is low. In other words,the isolation between the input and the output of the circuit on theside of the high output power last phase amplifying element 11 that isturned off can be further enhanced, and hence the oscillation can befurther suppressed.

According to Embodiment 4, the high frequency amplifier 100 illustratedin FIG. 9 includes the first matching circuit 34 disposed on the outputside of the high output power last phase amplifying element 11, thesecond matching circuit 35 disposed on the output side of the low outputpower last phase amplifying element 12 and the third matching circuit 36disposed on the post stage thereof. Therefore, it can be matched to thecharacteristic impedance of 50 ohms (Ω) in any cases of high and lowoutput powers, and hence characteristics such as the high output powerand high efficiency can be realized as the high frequency amplifier.

In addition, the impedance of the matching circuit on the turned-offamplifying element side viewed from the connection node 29 can besufficiently higher than the impedance of the matching circuit on theturned-on amplifying element side viewed from the connection node 29 inany cases of high and low output powers. Therefore, it is possible toprevent the amplified high frequency signal from passing around to thematching circuit on the turned-off amplifying element side, and hence aloss in the output matching circuit 15 can be reduced, wherebycharacteristics such as the high output power and high efficiency can berealized as the high frequency amplifier. Further, isolation between theinput and the output on the turned-off amplifying element side can beenhanced, and the oscillation due to the signal passing around theturned-off amplifying element can be suppressed.

Further, in the case where the output power is high, the output matchingcircuit 15 when the high output power last phase amplifying element 11is turned on is made up mainly of the low pass filter type matchingcircuit, and hence a loss in the output matching circuit 15 in the casewhere the output power is high can be reduced, whereby the highfrequency amplifier 100 can have higher output power as well as higherefficiency.

Further, the impedance Z17 of the first matching circuit 34 viewed fromthe connection node 29 can be further increased in the case where theoutput power is low. The loss generated by the high frequency signalpassing around to the first matching circuit 34 can be further reducedin the output matching circuit 15, and hence the output power and theefficiency can be further enhanced in the case where the output power islow. In addition, the isolation between the input and the output of thecircuit on the side of the high output power last phase amplifyingelement 11 that is turned off can be further enhanced, and hence theoscillation can be further suppressed.

Note that the circuit described in Embodiment 4 is made up mainly oflumped constant elements, but the serial inductor 25 may be made up of aserial line, the parallel capacitor 22 may be made up of an open stub,and the parallel inductor may be made up of a short stub. The amplifyingelements 11 and 12 are made up of a heterobipolar transistor (HBT), butmay be made up of another bipolar transistor or a field effecttransistor (FET) such as a metal-semiconductor FET (MESFET) or a highelectron mobility transistor (HEMT). Further, the collector (drain) biasapplying inductor 26 may be used instead of the collector (drain) biasline 23, and vice versa. Specifically, the collector (drain) bias line23 may be used instead of the collector (drain) bias applying inductor26. In addition, the collector (drain) bias line 23 and the collector(drain) bias applying inductor 26 work also as matching elements.

Embodiment 5

A high frequency amplifier according to Embodiment 5 of the presentinvention is described with reference to FIGS. 11 and 12A-12D. FIG. 11is a circuit diagram illustrating a structure of the high frequencyamplifier according to Embodiment 5 of the present invention.

In FIG. 11, a high frequency amplifier 100 according to Embodiment 5includes an input terminal 1, an output terminal 2, a collector (drain)bias terminal 4, a base (gate) bias setting terminal 5, and a modeswitching terminal 6.

In addition, the high frequency amplifier 100 includes a high outputpower last phase amplifying element (first amplifying element) 11, a lowoutput power last phase amplifying element (second amplifying element)12, two input matching circuits 13, an output matching circuit 15, andtwo base (gate) bias control circuits (first and second bias controlcircuits) 16. An element size of the high output power last phaseamplifying element 11 is larger than a size of the low output power lastphase amplifying element 12. Note that each of the two base (gate) biascontrol circuits 16 is connected to a power source terminal 28.

The output matching circuit 15 includes a first matching circuit 34, asecond matching circuit 35, and a third matching circuit 36. Note thatthe first and second matching circuits 34 and 35 are connected to thethird matching circuit 36 via a connection node 29.

The first matching circuit 34 includes a short stub including acollector (drain) bias line 23 and a bypass capacitor 24, a low passfilter type matching circuit 30, a high pass filter type matchingcircuit (first high pass filter type matching circuit) 27, and a serialinductor (first serial inductor) 25. In addition, the low pass filtertype matching circuit 30 includes the serial inductor 25 and a parallelcapacitor 22. The high pass filter type matching circuit 27 includes aserial capacitor 17 and a parallel inductor 18. Note that an end of eachof the bypass capacitor 24, the parallel capacitor 22 and the parallelinductor 18 is connected to a ground 19.

The second matching circuit 35 includes a high pass filter type matchingcircuit (second high pass filter type matching circuit) 27 and a serialinductor (second serial inductor) 25. In addition, the high pass filtertype matching circuit 27 includes a collector (drain) bias applyinginductor 26, the bypass capacitor 24, the serial capacitor 17, and theparallel inductor 18. Note that an end of each of the bypass capacitor24 and the parallel inductor 18 is connected to the ground 19.

The third matching circuit 36 includes the serial capacitor 17.

The high frequency amplifier 100 of Embodiment 5 illustrated in FIG. 11is different from the high frequency amplifier 100 of Embodiment 3illustrated in FIG. 7 only in that the first matching circuit 34 is madeup of the short stub including the collector (drain) bias line 23 andthe bypass capacitor 24, the low pass filter type matching circuit 30including the serial inductor 25 and the parallel capacitor 22, the highpass filter type matching circuit 27 including the serial capacitor 17and the parallel inductor 18, and the serial inductor 25.

Next, an operation of the high frequency amplifier according toEmbodiment 5 is described with reference to the drawings. FIGS. 12A-12Dare Smith charts illustrating impedance of the output matching circuitof the high frequency amplifier according to Embodiment 5 of the presentinvention.

A flow of the signal in the high frequency amplifier 100, a method ofapplying the bias, and the conditions required to the output matchingcircuit 15 are the same as those in the above-mentioned embodiments, andhence descriptions thereof are omitted.

An operation of the output matching circuit 15 of the high frequencyamplifier 100 illustrated in FIG. 11 is described. FIGS. 12A and 12Billustrate loci of impedance of the first matching circuit 34 viewedfrom the connection node 29 in the case where the output power is highand in the case where it is low, respectively, with full line arrows.FIGS. 12C and 12D illustrate loci of impedance of the second matchingcircuit 35 viewed from the connection node 29 in the case where theoutput power is high and in the case where it is low, respectively, withfull line arrows. In addition, a locus of impedance of the circuit fromthe connection node 29 to the output terminal 2 is also illustrated witha dotted line arrow. In FIGS. 12A-12D, Zout1, Z11, Z12, Z13, Z14, Z15,Z16, Zout2, Z21, Z22, Z23, Z24, and Zout respectively denote impedancesviewed from positions indicated on the circuit diagram of FIG. 11.

FIG. 12A is a diagram illustrating impedance of the first matchingcircuit 34 viewed from the connection node 29 in the case where theoutput power is high. FIG. 12B is a diagram illustrating impedance ofthe first matching circuit 34 viewed from the connection node 29 in thecase where the output power is low. FIG. 12C is a diagram illustratingimpedance of the second matching circuit 35 viewed from the connectionnode 29 in the case where the output power is high. FIG. 12D is adiagram illustrating impedance of the second matching circuit 35 viewedfrom the connection node 29 in the case where the output power is low.

With reference to FIGS. 12A and 12D, the structure of the first matchingcircuit 34 of Embodiment 5 is difference from that of the first matchingcircuit 34 of Embodiment 3. However, it is understood that Z16 that isimpedance (first impedance) of the first matching circuit 34 viewed fromthe connection node 29 in the case where the output power is high issubstantially the same as Z24 that is impedance (second impedance) ofthe second matching circuit 35 viewed from the connection node 29 in thecase where the output power is low. Therefore, the above-mentioned“first condition” can be satisfied. Thus, the output impedance Zout ofthe high frequency amplifier 100 can be matched to 50 ohms (Ω) by thethird matching circuit 36 in any case when the amplifying elements 11and 12 are switched in accordance with a level of the output power.Therefore, the high frequency amplifier 100 can realize characteristicssuch as high output power and high efficiency in any cases where theoutput power is high and where it is low.

With reference to FIGS. 12C and 12A, the structure of the first matchingcircuit 34 of Embodiment 5 is difference from that of the first matchingcircuit 34 of Embodiment 3. However, it is understood that Z24 that isimpedance (second impedance) of the second matching circuit 35 viewedfrom the connection node 29 in the case where the output power is highis sufficiently higher than Z16 that is impedance (first impedance) ofthe first matching circuit 34 viewed from the connection node 29 in thecase where the output power is high. Therefore, the above-mentioned“second condition” can be satisfied. Thus, the loss due to the highfrequency signal that passes around to the second matching circuit 35 inthe output matching circuit 15 can be reduced, and hence characteristicssuch as the high output power and high efficiency can be realized in thecase where the output power is high. At the same time, the highfrequency signal does not pass around to the second matching circuit 35,and hence the oscillation generated by a feedback of the high frequencysignal amplified by the high output power last phase amplifying element11 to the input side via the low output power last phase amplifyingelement 12 that is turned off can be suppressed in the case where theoutput power is high. In other words, isolation between the input andthe output of the circuit on a side of the low output power last phaseamplifying element 12 that is turned off can be enhanced so that theoscillation can be suppressed.

Here, a method of increasing Z24 that is impedance of the secondmatching circuit 35 viewed from the connection node 29 in the case wherethe output power is high is described. The output impedance Zout2 whenthe low output power last phase amplifying element 12 is turned off iscapacitive impedance as illustrated in FIG. 12C, and hence the secondmatching circuit 35 can increase the impedance viewed from theconnection node 29 by using a high pass filter type matching elementsuch as the serial capacitor 17 or the parallel inductor 18. In view ifthis, the second circuit 35 includes the serial inductor 25 provided ata position that is closest to the connection node 29 from the fact thatthe impedance (Z23) is inductive. In this way, it is necessary todispose the high pass filter type matching circuit 27 and the serialinductor 25 on the connection node 29 side of the second matchingcircuit 35.

The structure of the first matching circuit 34 of Embodiment 5 isdifference from that of the first matching circuit 34 of Embodiment 3.However, it is understood from FIGS. 12B and 12D that Z16 that isimpedance (first impedance) of the first matching circuit 34 viewed fromthe connection node 29 in the case where the output power is low issufficiently higher than Z24 that is impedance (second impedance) of thesecond matching circuit 35 viewed from the connection node 29 in thecase where the output power is low. Therefore, the above-mentioned“third condition” can be satisfied. Thus, the loss generated by the highfrequency signal passing around to the first matching circuit 34 in theoutput matching circuit 15 can be reduced, and hence characteristicssuch as the high output power and high efficiency can be realized in thecase where the output power is low. At the same time, the high frequencysignal does not pass around to the first matching circuit 34, and hencethe oscillation generated by a feedback of the high frequency signalamplified by the low output power last phase amplifying element 12 tothe input side via the high output power last phase amplifying element11 that is turned off can be suppressed in the case where the outputpower is low. In other words, isolation between the input and the outputof the circuit on a side of the high output power last phase amplifyingelement 11 that is turned off can be enhanced, and hence the oscillationcan be suppressed.

Here, a method of increasing the impedance Z16 of the first matchingcircuit 34 viewed from the connection node 29 in the case where theoutput power is low is described. The output impedance Zout1 when thehigh output power last phase amplifying element 11 is turned off iscapacitive impedance as illustrated in FIG. 12B, and hence the firstmatching circuit 34 can increase the impedance (Z11) by providing ashort stub. This short stub is made up of the collector (drain) biasline 23 and the bypass capacitor 24. However, the impedance (Z13) isdecreased because of the low pass filter type matching circuit 30 thatis provided for matching in the case where the high output power lastphase amplifying element 11 is turned on, but the impedance iscapacitive one because the low pass filter type matching circuit 30 is asingle stage. Therefore, the first matching circuit 34 can increase theimpedance viewed from the connection node 29 by using the high passfilter type matching element such as the serial capacitor 17 or theparallel inductor 18. In view of this, the impedance is inductive, andhence the serial inductor 25 of the first matching circuit 34 isdisposed at the position that is closest to the connection node 29. Inthis way, it is necessary to dispose the high pass filter type matchingcircuit 27 and the serial inductor 25 on the connection node 29 side ofthe first matching circuit 34.

In addition, the high frequency amplifier 100 according to Embodiment 5is compared with the high frequency amplifier 100 according to theabove-mentioned Embodiment 3 as follows. The low pass filter typematching circuit 30 of Embodiment 3 has a two-stage structure while thelow pass filter type matching circuit 30 of Embodiment 5 is a singlestage, and the high pass filter type matching circuit 27 is disposed atthe position close to the connection node 29 instead of the single stagelow pass filter type matching circuit. Therefore, the impedance Z16 ofthe first matching circuit 34 on the side of the high output power lastphase amplifying element 11 to be turned off viewed from the connectionnode 29 in the case where the output power is low can be higher thanthat in Embodiment 3 illustrated in FIG. 8B. Therefore, a loss generatedby the high frequency signal passing around to the first matchingcircuit 34 in the output matching circuit 15 can be further reduced inthe case where the output power is low, and hence the output power andthe efficiency can be further enhanced in the case where the outputpower is low. At the same time, the high frequency signal does not passaround to the first matching circuit 34, and hence the oscillationgenerated by a feedback of the high frequency signal amplified by thelow output power last phase amplifying element 12 to the input side viathe high output power last phase amplifying element 11 that is turnedoff can be further suppressed in the case where the output power is low.In other words, the isolation between the input and the output of thecircuit on the side of the high output power last phase amplifyingelement 11 that is turned off can be further enhanced, and hence theoscillation can be further suppressed.

According to Embodiment 5, the high frequency amplifier 100 illustratedin FIG. 11 includes the first matching circuit 34 disposed on the outputside of the high output power last phase amplifying element 11, thesecond matching circuit 35 disposed on the output side of the low outputpower last phase amplifying element 12 and the third matching circuit 36disposed on the post stage thereof. Therefore, it can be matched to thecharacteristic impedance of 50 ohms (Ω) in any cases of high and lowoutput powers, and hence characteristics such as the high output powerand high efficiency can be realized as the high frequency amplifier.

In addition, the impedance of the matching circuit on the turned-offamplifying element side viewed from the connection node 29 can besufficiently higher than the impedance of the matching circuit on theturned-on amplifying element side viewed from the connection node 29 inany cases of high and low output powers. Therefore, it is possible toprevent the amplified high frequency signal from passing around to thematching circuit on the turned-off amplifying element side, and hence aloss in the output matching circuit 15 can be reduced, wherebycharacteristics such as the high output power and high efficiency can berealized as the high frequency amplifier. Further, isolation between theinput and the output on the turned-off amplifying element side can beenhanced, and the oscillation due to the signal passing around theturned-off amplifying element can be suppressed.

Further, the impedance of the first matching circuit 34 viewed from theconnection node 29 can be further increased in the case where the outputpower is low. The loss generated by the high frequency signal passingaround to the first matching circuit 34 can be further reduced in theoutput matching circuit 15, and hence the output power and theefficiency can be further enhanced in the case where the output power islow. In addition, the isolation between the input and the output of thecircuit on the side of the high output power last phase amplifyingelement 11 that is turned off can be further enhanced, and hence theoscillation can be further suppressed.

Note that the circuit described in Embodiment 5 is made up mainly oflumped constant elements, but the serial inductor 25 may be made up of aserial line, the parallel capacitor 22 may be made up of an open stub,and the parallel inductor may be made up of a short stub. The amplifyingelements 11 and 12 are made up of a heterobipolar transistor (HBT), butmay be made up of another bipolar transistor or a field effecttransistor (FET) such as a metal-semiconductor FET (MESFET) or a highelectron mobility transistor (HEMT). Further, the collector (drain) biasapplying inductor 26 may be used instead of the collector (drain) biasline 23, and vice versa. Specifically, the collector (drain) bias line23 may be used instead of the collector (drain) bias applying inductor26. In addition, the collector (drain) bias line 23 and the collector(drain) bias applying inductor 26 work also as matching elements.

Embodiment 6

A high frequency amplifier according to Embodiment 6 of the presentinvention is described with reference to FIG. 13. FIG. 13 is a circuitdiagram illustrating a structure of the high frequency amplifieraccording to Embodiment 6 of the present invention.

In FIG. 13, a high frequency amplifier 100 according to Embodiment 6includes an input terminal 1, an output terminal 2, a collector (drain)bias terminal 4, a base (gate) bias setting terminal 5, and a modeswitching terminal 6.

In addition, the high frequency amplifier 100 includes a high outputpower last phase amplifying element (first amplifying element) 11, a lowoutput power last phase amplifying element (second amplifying element)12, two input matching circuits 13, an output matching circuit 15, andtwo base (gate) bias control circuits (first and second bias controlcircuits) 16. An element size of the high output power last phaseamplifying element 11 is larger than a size of the low output power lastphase amplifying element 12. Note that each of the two base (gate) biascontrol circuits 16 is connected to a power source terminal 28.

The output matching circuit 15 includes a first matching circuit 34, asecond matching circuit 35, a third matching circuit 36 and a switch 31.Note that the first and second matching circuits 34 and 35 are connectedto the third matching circuit 36 via a connection node 29 and via theswitch 31 and the connection node 29, respectively.

The first matching circuit 34 includes a short stub made up of acollector (drain) bias line 23 and a bypass capacitor 24, a low passfilter type matching circuit 30, and a serial inductor (first serialinductor) 25. In addition, the low pass filter type matching circuit 30includes two stages of circuits made up of the serial inductor 25 and aparallel capacitor 22. Note that an end of each of the bypass capacitor24 and parallel capacitor 22 is connected to a ground 19.

The second matching circuit 35 includes a high pass filter type matchingcircuit 27 and a serial inductor (second serial inductor) 25. Inaddition, the high pass filter type matching circuit 27 includes acollector (drain) bias applying inductor 26, the bypass capacitor 24,and a serial capacitor 17. Note that an end of the bypass capacitor 24is connected to the ground 19.

The third matching circuit 36 includes the serial capacitor 17.

The high frequency amplifier 100 illustrated in FIG. 13 is differentfrom the high frequency amplifier 100 illustrated in FIG. 7 only in thatthe switch 31 made up of a diode 32 is disposed between the secondmatching circuit 35 and the connection node 29.

Next, an operation of the high frequency amplifier according toEmbodiment 6 is described with reference to the drawings.

Only the part that is different from the high frequency amplifier 100 ofEmbodiment 3 illustrated in FIG. 7 is described. The high frequencyamplifier 100 according to Embodiment 6 includes the switch 31 made upof the diode 32 disposed between the second matching circuit 34 and theconnection node 29. This switch 31 is controlled by the voltage that isapplied to the mode switching terminal 6 so as to be turned off in thecase where the output power is high and to be turned on in the casewhere the output power is low.

Therefore, the impedance of the second matching circuit 35 viewed fromthe connection node 29 in the case where the output power is high can befurther increased because the switch 31 is turned off. Thus, the lossdue to the high frequency signal that passes around to the secondmatching circuit 35 in the output matching circuit 15 can be reduced,and hence characteristics such as the high output power and highefficiency can be realized in the case where the output power is high.At the same time, the high frequency signal does not pass around to thesecond matching circuit 35, and hence the oscillation generated by afeedback of the high frequency signal amplified by the high output powerlast phase amplifying element 11 to the input side via the low outputpower last phase amplifying element 12 that is turned off can be furthersuppressed in the case where the output power is high. In other words,isolation between the input and the output of the circuit on a side ofthe low output power last phase amplifying element 12 that is turned offcan be further enhanced so that the oscillation can be furthersuppressed.

According to Embodiment 6, the high frequency amplifier 100 of FIG. 13can have the following effect in addition to the effect of the highfrequency amplifier 100 of Embodiment 3 illustrated in FIG. 7. The lossgenerated by the high frequency signal that passes around to the secondmatching circuit 35 in the output matching circuit 15 can be reduced inthe case where the output power is high, and hence characteristics suchas the high output power and high efficiency can be realized in the casewhere the output power is high. At the same time, the high frequencysignal does not pass around to the second matching circuit 35, and hencethe oscillation generated by the feedback of the high frequency signalamplified by the high output power last phase amplifying element 11 tothe input side via the low output power last phase amplifying element 12that is turned off can be further suppressed in the case where theoutput power is high. In other words, the isolation between the inputand the output of the circuit on the side of the low output power lastphase amplifying element 12 that is turned off can be further enhanced,and hence the oscillation can be further suppressed.

The case where the switch 31 made up of the diode 32 is applied toEmbodiment 3 (FIG. 7) is described in Embodiment 6, and the same effectcan be obtained in the case where it is applied to Embodiment 1 (FIG.1), Embodiment 2 (FIG. 5), Embodiment 4 (FIG. 9) or Embodiment 5 (FIG.11). In addition, the case where the diode 32 is used as the switch 31is described in Embodiment 6, but another switch such as an FET switchor a mechanical switch may be used as the switch 31.

The amplifying elements 11 and 12 are made up of a heterobipolartransistor (HBT), but may be made up of another bipolar transistor or afield effect transistor (FET) such as a metal-semiconductor FET (MESFET)or a high electron mobility transistor (HEMT). Further, the collector(drain) bias applying inductor 26 may be used instead of the collector(drain) bias line 23, and vice versa. Specifically, the collector(drain) bias line 23 may be used instead of the collector (drain) biasapplying inductor 26. In addition, the collector (drain) bias line 23and the collector (drain) bias applying inductor 26 work also asmatching elements.

Embodiment 7

A high frequency amplifier according to Embodiment 7 of the presentinvention is described with reference to FIG. 14. FIG. 14 is a circuitdiagram illustrating a structure of the high frequency amplifieraccording to Embodiment 7 of the present invention.

In FIG. 14, a high frequency amplifier 100 according to Embodiment 7includes an input terminal 1, an output terminal 2, a collector (drain)bias terminal 4, a base (gate) bias setting terminal 5, and a modeswitching terminal 6.

In addition, the high frequency amplifier 100 includes a high outputpower last phase amplifying element (first amplifying element) 11, a lowoutput power last phase amplifying element (second amplifying element)12, two input matching circuits 13, an output matching circuit 15, twobase (gate) bias control circuits (first and second bias controlcircuits) 16, and a grounded base (gate) transistor (third amplifyingelement) 33. An element size of the high output power last phaseamplifying element 11 is larger than a size of the low output power lastphase amplifying element 12. Note that each of the two base (gate) biascontrol circuits 16 is connected to a power source terminal 28.

The output matching circuit 15 includes a first matching circuit 34, asecond matching circuit 35, and a third matching circuit 36. Note thatthe first and second matching circuits 34 and 35 are connected to thethird matching circuit 36 via a connection node 29.

The first matching circuit 34 includes a short stub made up of acollector (drain) bias line 23 and a bypass capacitor 24, a low passfilter type matching circuit 30, and a serial inductor (first serialinductor) 25. In addition, the low pass filter type matching circuit 30includes two stages of circuits made up of the serial inductor 25 and aparallel capacitor 22. Note that an end of each of the bypass capacitor24 and parallel capacitor 22 is connected to a ground 19.

The second matching circuit 35 includes a high pass filter type matchingcircuit 27 and a serial inductor (second serial inductor) 25. Inaddition, the high pass filter type matching circuit 27 includes acollector (drain) bias applying inductor 26, the bypass capacitor 24,and a serial capacitor 17. Note that an end of the bypass capacitor 24is connected to the ground 19.

The third matching circuit 36 includes the serial capacitor 17.

The high frequency amplifier 100 illustrated in FIG. 14 is differentfrom the high frequency amplifier 100 illustrated in FIG. 7 only in thatthe grounded base (gate) transistor 33 is inserted, which is connectedin cascode to the output side of the low output power last phaseamplifying element 12.

Next, an operation of the high frequency amplifier according toEmbodiment 7 is described with reference to the drawings.

Only the part that is different from the high frequency amplifier 100 ofEmbodiment 3 illustrated in FIG. 7 is described. In the high frequencyamplifier 100 of Embodiment 7 illustrated in FIG. 14, the grounded base(gate) transistor 33 is inserted to the output side of the low outputpower last phase amplifying element 12. A base voltage of the groundedbase (gate) transistor 33 is supplied from the base (gate) bias controlcircuit (second bias control circuit) 16. Then, the base (gate) biascontrol circuit 16 controls so that the grounded base (gate) transistor33 is turned off in the case where the output power is high while thegrounded base (gate) transistor 33 is turned on in the case where theoutput power is low, by the voltage from the mode switching terminal 6.

Therefore, the signal passing around to the low output power last phaseamplifying element 12 via the second matching circuit 35 is cut off bythe turned-off grounded base (gate) transistor 33 in the case where theoutput power is high. Thus, the oscillation generated by the feedback ofthe high frequency signal amplified by the high output power last phaseamplifying element 11 to the input side via the low output power lastphase amplifying element 12 that is turned off can be furthersuppressed. In other words, the isolation between the input and theoutput of the circuit on the side of the low output power last phaseamplifying element 12 that is turned off can be further enhanced, andhence the oscillation can be further suppressed.

According to Embodiment 7, the high frequency amplifier 100 illustratedin FIG. 14 can have the following effect in addition to the effect ofthe high frequency amplifier 100 of Embodiment 3 illustrated in FIG. 7.The signal passing around to the low output power last phase amplifyingelement 12 via the second matching circuit 35 can be cut off by theturned-off grounded base (gate) transistor 33 in the case where theoutput power is high, and hence the oscillation generated by thefeedback of the high frequency signal amplified by the high output powerlast phase amplifying element 11 to the input side via the low outputpower last phase amplifying element 12 that is turned off can be furthersuppressed. In other words, the isolation between the input and theoutput of the circuit on the side of the low output power last phaseamplifying element 12 that is turned off can be further enhanced, andhence the oscillation can be further suppressed.

The case where the grounded base (gate) transistor 33 is applied to thehigh frequency amplifier 100 of Embodiment 3 illustrated in FIG. 7 isdescribed in Embodiment 7, and the same effect can be obtained in thecase where it is applied to the high frequency amplifier 100 ofEmbodiment 1 (FIG. 1), Embodiment 2 (FIG. 5), Embodiment 4 (FIG. 9),Embodiment 5 (FIG. 11) or Embodiment 6 (FIG. 13).

The amplifying elements 11 and 12 are made up of a heterobipolartransistor (HBT), but may be made up of another bipolar transistor or afield effect transistor (FET) such as a metal-semiconductor FET (MESFET)or a high electron mobility transistor (HEMT). Further, the collector(drain) bias applying inductor 26 may be used instead of the collector(drain) bias line 23, and vice versa. Specifically, the collector(drain) bias line 23 may be used instead of the collector (drain) biasapplying inductor 26. In addition, the collector (drain) bias line 23and the collector (drain) bias applying inductor 26 work also asmatching elements.

Embodiment 8

A high frequency amplifier according to Embodiment 8 of the presentinvention is described with reference to FIG. 15. FIG. 15 is a circuitdiagram illustrating a structure of the high frequency amplifieraccording to Embodiment 8 of the present invention.

In FIG. 15, a high frequency amplifier 100 according to Embodiment 8includes an input terminal 1, an output terminal 2, a collector (drain)bias terminal 4, a base (gate) bias setting terminal 5, and a modeswitching terminal 6.

In addition, the high frequency amplifier 100 includes a high outputpower last phase amplifying element (first amplifying element) 11, a lowoutput power last phase amplifying element (second amplifying element)12, two input matching circuits 13, an output matching circuit 15, twobase (gate) bias control circuits (first and second bias controlcircuits) 16, a high output power pre-amplifying element (thirdamplifying element) 8, a low output power pre-amplifying element (fourthamplifying element) 9, and two interstage matching circuits (first andsecond interstage matching circuits) 14. An element size of the highoutput power last phase amplifying element 11 is larger than a size ofthe low output power last phase amplifying element 12. Note that each ofthe two base (gate) bias control circuits 16 is connected to a powersource terminal 28.

The output matching circuit 15 includes a first matching circuit 34, asecond matching circuit 35, and a third matching circuit 36. Note thatthe first and second matching circuits 34 and 35 are connected to thethird matching circuit 36 via a connection node 29.

The first matching circuit 34 includes a short stub made up of acollector (drain) bias line 23 and a bypass capacitor 24, a low passfilter type matching circuit 30, and a serial inductor (first serialinductor) 25. In addition, the low pass filter type matching circuit 30includes two stages of circuits made up of the serial inductor 25 and aparallel capacitor 22. Note that an end of each of the bypass capacitor24 and parallel capacitor 22 is connected to a ground 19.

The second matching circuit 35 includes a high pass filter type matchingcircuit 27 and a serial inductor (second serial inductor) 25. Inaddition, the high pass filter type matching circuit 27 includes acollector (drain) bias applying inductor 26, the bypass capacitor 24,and a serial capacitor 17. Note that an end of the bypass capacitor 24is connected to the ground 19.

The third matching circuit 36 includes the serial capacitor 17.

The high frequency amplifier 100 of Embodiment 8 illustrated in FIG. 15is different from the high frequency amplifier 100 of Embodiment 3illustrated in FIG. 7 only in that the high output power pre-amplifyingelement 8, the low output power pre-amplifying element 9 and the twointerstage matching circuits 14 are added, and hence the amplifyingelement to be switched has a two-stage structure.

Next, an operation of the high frequency amplifier according toEmbodiment 8 is described with reference to the drawings.

Only the part that is different from the high frequency amplifier 100 ofEmbodiment 3 illustrated in FIG. 7 is described. The collector (drain)bias is supplied to the high output power pre-amplifying element 8 andto the low output power pre-amplifying element 9 via the interstagematching circuits 14 from the collector (drain) bias terminal 4. Thebase (gate) bias is supplied to the high output power pre-amplifyingelement 8 and to the low output power pre-amplifying element 9 from thetwo base (gate) bias control circuits 16, respectively.

According to Embodiment 8, the high frequency amplifier 100 illustratedin FIG. 15 can obtain a much higher gain in addition to the effect ofthe high frequency amplifier 100 of Embodiment 3 illustrated in FIG. 7.In addition, considering as two stages of amplifiers, not only the lastphase amplifying element 12 but also the pre-amplifying element 9 has asmall size in the case where the output power is low. Therefore, powerconsumption can be further reduced, and hence higher efficiencycharacteristics can be realized.

The case where the two stages of the amplifying elements are applied tothe high frequency amplifier of Embodiment 3 illustrated in FIG. 7 isdescribed as the high frequency amplifier 100 of Embodiment 8illustrated in FIG. 15, and the same effect can be obtained in the casewhere it is applied to the high frequency amplifier 100 of Embodiment 1(FIG. 1), Embodiment 2 (FIG. 5), Embodiment 4 (FIG. 9), Embodiment 5(FIG. 11), Embodiment 6 (FIG. 13) or Embodiment 7 (FIG. 14).

The amplifying elements 11 and 12 are made up of a heterobipolartransistor (HBT), but may be made up of another bipolar transistor or afield effect transistor (FET) such as a metal-semiconductor FET (MESFET)or a high electron mobility transistor (HEMT). Further, the collector(drain) bias applying inductor 26 may be used instead of the collector(drain) bias line 23, and vice versa. Specifically, the collector(drain) bias line 23 may be used instead of the collector (drain) biasapplying inductor 26. In addition, the collector (drain) bias line 23and the collector (drain) bias applying inductor 26 work also asmatching elements.

1. A high frequency amplifier, comprising: a first amplifying elementfor amplifying a high frequency signal input from an input terminal; asecond amplifying element for amplifying the high frequency signal,which is connected in parallel to the first amplifying element and has asmaller element size than the first amplifying element has; a first biascontrol circuit for turning on and off the first amplifying elementbased on a mode switching voltage for switching between a case where anoutput power is high and a case where the output power is low; a secondbias control circuit for turning on and off the second amplifyingelement based on the mode switching voltage; and an output matchingcircuit connected to output sides of the first amplifying element andthe second amplifying element, the output matching circuit comprising: afirst matching circuit connected to the output side of the firstamplifying element; a second matching circuit connected to the outputside of the second amplifying element; and a third matching circuitconnected between an output terminal and a connection node of the outputsides of the first matching circuit and the second matching circuit,which is matched to 50 ohms, wherein: the first matching circuitcomprises: a low pass filter type matching circuit connected to theoutput side of the first amplifying element; and a first serial inductorconnected to the low pass filter type matching circuit; the secondmatching circuit comprises: a high pass filter type matching circuitconnected to the output side of the second amplifying element; and asecond serial inductor connected to the high pass filter type matchingcircuit; a first impedance of the first matching circuit viewed from theconnection node in the case where the output power is high that is acase where the first amplifying element is turned on while the secondamplifying element is turned off is substantially the same as a secondimpedance of the second matching circuit viewed from the connection nodein the case where the output power is low that is a case where thesecond amplifying element is turned on while the first amplifyingelement is turned off; the second impedance of the second matchingcircuit viewed from the connection node is higher than the firstimpedance of the first matching circuit viewed from the connection nodein the case where the output power is high that is the case where thefirst amplifying element is turned on while the second amplifyingelement is turned off; and the first impedance of the first matchingcircuit viewed from the connection node is higher than the secondimpedance of the second matching circuit viewed from the connection nodein the case where the output power is low that is the case where thesecond amplifying element is turned on while the first amplifyingelement is turned off.
 2. A high frequency amplifier according to claim1, wherein the low pass filter type matching circuit is connected to theoutput side of the first amplifying element, and comprises a two-stagecircuit of a third serial inductor and a first parallel capacitor.
 3. Ahigh frequency amplifier according to claim 2, wherein the low passfilter type matching circuit further comprises a second parallelcapacitor connected between the output side of the first amplifyingelement and the two-stage circuit.